SV-POW! … All sauropod vertebrae, except when we're talking about Open Access

SV-POW! showdown: stegosaurs vs sauropods

February 26, 2009

New hotness out today: Miragaia, a new long-necked stegosaur from the Late Jurassic of Portugal (Mateus et al. 2009). What is “long-necked” for a stegosaur? In this case, well over a meter! That may not sound too impressive for those of you who have gotten complacent about 10-meter-plus sauropod necks, but it’s a big deal. Miragaia is described as a sauropod mimic, and with good reason: its body proportions are not that different than those of a basal sauropod.

The number of ways to increase the proportional length of the neck are limited: you can add cervicals, or recruit dorsals into the neck, or make the individual vertebrae longer, or do some combination of the above. In sauropods, different clades took different routes. Brachiosaurids kept a fairly primitive cervical count of 13 but made the individual vertebrae crazy long. Diplodocids recruited dorsals into the neck, and some (like Barosaurus and Supersaurus) also made the vertebrae crazy long. Mamenchisaurids and Euhelopus added cervicals (independently), up to a total of 17 or more, and some (like Omeisaurus)–are you ready for it?–also made the vertebrae crazy long.

In general, stegosaurstook an evolutionary walk through Door Number 2: turning dorsals into cervicals. Mateus et al. (2009) show this nicely in a table; the number of presacrals (cervicals plus dorsals) in stegosaurs stays about the same, between 25 and 27, but between the basal Huayangosaurus and the derived Stegosaurus 3 or 4 dorsals go forward to play for the other team. Is dorsal recruitment sufficient to explain the long neck of Miragaia? Hard to say, since the dorsal series has not been found. But Miragaia‘s count of 17 cervicals is significantly more than Stegosaurus‘s 13. If Miragaia didn’t add any cervicals but only recruited dorsals, it would have had only 9 of the latter. That’s not impossible–Barosaurus did that very thing–but it’s weird, and extreme. As Mateus et al. (2009:p. 4) state, “Miragaia possessed more cervical vertebrae than any other non-avian archosaur, except the Chinese sauropods Mamenchisaurus, Omeisaurus and Euhelopus, also with 17″. And yet the individual vertebrae are pretty short, no longer than in your not-exactly-averageStegosaurus.

I couldn’t resist pitting Miragaia, the longest-necked stegosaur (so far!) against Brachytrachelopan, the shortest-necked sauropod (so far!). Miragaia is stolen from Mateus et al. (2009:fig. 1a), and Brachytrachelopan from Rauhut et al. (2005:fig. 1a). Both critters come with the 1 meter scale bars from their respective figures. I’m in there for scale, too, at 6’2″ or 1.88 meters. Sauroposeidon looms in the background, just to keep things in perspective. The entire neck of Miragaia might have been about as long as one of the middle cervicals of Sauroposeidon or Supersaurus.

Re: Adam’s suggestion that Dacentrurus and Miragaia might be the same thing, this IS testable as, in fact, there is some overlap between the two taxa, just not between the holotypes. The Murteiras Ravine Dacentrurus from Portugal includes cervical and dorsal vertebrae, for example, as does the Cerrito del Olmo specimen. These specimens were confidently assigned to Dacentrurus by Maidment et al. (2008): given that Maidment is on the authorship of the Miragaia paper, I’d be pretty confident that she and her co-authors have made the required comparisons. If you’re out there, Susannah, feel free to confirm :)

That’s not a super helpful comparison, because the authors admit in the paper that the referred specimen “possesses none of the autapomorphies of M. longicollum“, and is only referred to the taxon because “its skeletal remains were found in close proximity to those of the holotype.”

Randy, I don’t quite follow. What I’m saying is that there is indeed overlap between Miragaia and Dacentrurus: the Miragaia holotype includes cervical vertebrae, while specimens referred to D. armatus do too. You seem to be talking about the referred Miragaia specimen, but this is not relevant to the issue at hand (viz, possible synonymy of Miragaia and Dacentrurus).

Fair enough – I misread your post. Nonetheless, it depends on the referral of the Murteiras Ravine material to Dacentrurus; right now we can’t compare holotype to holotype. Even with this, as Adam notes, all but one of the characters that differ between the two taxa are “…continuously variable characters (e.g. ratio of distal width of the humerus to its length)and in most cases the difference was slight so that in more traditional discrete character state coding these features might be given the same state. The one discrete character difference in the matrix concerned the robustness of the dorsal plates but once again the known plates of Miragaia are from the front of the animal while those known from Dacenturus come from further back so we may not be comparing the same thing.” I’m just surprised the authors don’t spend more time justifying the separation of the two taxa, especially when they come out as sister groups in the phylogenetic analysis.

That being said, the taxonomy of the material has no bearing of its significance to the main point of the paper, which is the very cool neck elongation trends, etc.

I’d do the numbers myself, but I’m not sure what counts as ‘neck’ and what doesn’t. Using a naive non-scientific definition (just measuring the narrow part on the scale drawings) it’s almost exactly 1:8 for Brachytrachelopan and a titch over 1:5 for Miragaia.

What about the authors claiming credit for the name Dacentrurinae? That should be credited to Olshevsky and Ford, who named Dacentruridae in 1993.

Olshevsky and Ford, 1993. The origin and evolution
of the stegosaurs. Gakken Mook. 4,65-103 (in Japanese).

Note Mateus et al. list the authors for the other names they use (Stegosauria, Ornithischia, etc.), even though they view these as clades with phylogenetic definitions as well. So just because they call it a “new clade” instead of a new subfamily doesn’t mean they get to credit the name to themselves, just like they don’t credit Stegosauridae to Sereno, even though he was the first to define it as a clade.

I am puzzled by that while Stegosaurus got a major overhaul and now includes a wide variety of morphologies, the same conservative approach was not taken on this new genus. Although the type material by itself appears very distinctive, it would not surprise me if some, if not all, of the Portugese Dacentrurus would be referrable to this animal. However, I would be hesitant to subsume the English Dacentrurus material in here as well, mainly because of the spatial difference. Admittingly, I need to catch up on the relevant literature, but it would be something to look into.

However, what I am particularly interested in is the implications for niche partitioning of Late Jurassic European dinosaurs. Based on this material, it seems that this genus of stegosaur was occupying a similar niche as that of the sauropod Dicraeosaurus in Tendaguru, Africa: low-medium browser. Clearly, this niche was available in the Lourinhana Formation before Miragaia evolved and I am surprised there was no Dicraeosaurid to step up the plate. Considering the increasing body of evidence that there was considerable megafauna exchange between the “European”, “American”, and “African” continent, where is the Portugese Dicraeosaurid?

Why did the new long-necked stegosaur get split into its own genus while other stegosaurs have been lumped into Stegosaurus? Simply because some workers are lumpers and some are splitters: we all have our genericometers calibrated differently. All this unpleasantness could be so easily avoided if only everyone would calibrate to _my_ level.

For what it’s worth, Matt and I are of the opinion that we as a community have been dramatically underestimating the diversity of the Morrison Formation sauropods; and since that’s the formation we know best, we don’t rule out the possibility that the same is true elsewhere. But no doubt for every one of us who suspects that “Apatosaurus” (= Brontosaurus) excelsus should be generically separate from Apatosaurus ajax, there is another who thinks that Apatosaurus and Diplodocus should be two species within the same genus.

(That’s one of the reasons that the Linnaean binomial as a curse rather than the brilliant invention that it’s usually painted as.)

Minor point on Mike’s last message: the inconsistency that Rutger is referring to (viz, where Miragaia and Dacentrurus are kept apart, while Hesperosaurus and Wuerhosaurus are sunk into Stegosaurus) probably isn’t due to ‘different genericometer calibrations’ because some of the authors involved ARE THE SAME!

A new monotypic genus vs a new species in an existing genus express exactly the same amount of diversity. When someone says that a taxon is different enough to be put in a new genus, they are talking about disparity, not diversity. My personal opinion is there’s really no reason to go around creating new genera if the existing genus is monophyletic.

Randy, thanks for the much-needed clarification on the distinction between diversity and disparity — which, believe it or not, I DO understand, but I admit I am often sloppy about.

However, it’s not quite true that “A new monotypic genus vs a new species in an existing genus express exactly the same amount of diversity”: the former implies more diversity because it expresses the author’s feeling that there may well be a whole complex of new species out there. Remember that diversity is a human artifact in the first place: whether we are counting “species-level” or “generic-level” diversity, the choice of whether or not two individuals belong to the same species, or to the same genus, is ultimately a matter of judgement and taste rather than of fact (and that goes double for palaeotaxonomy). Seen in that light, it’s clear that the choice to erect a genus indicates as stronger statement that erecting a species.

The bottom line here is that taxonomy just IS an subjective process and there is no way to “fix” that — apparently objective rules like “there is no reason to create a new genus if the existing genus is monophyletic” just make things worse. I think there are only three possible responses to the subjectivity of taxonomy: either we accept and embrace it; or we throw taxonomy out all together and talk in specimen-numbers; or (the most popular option) we stick out fingers in our ears and sing LA LA LA CAN’T HEAR YOU. I’m not fond of that one.

Mike said: “…the former implies more diversity because it expresses the author’s feeling that there may well be a whole complex of new species out there.”

Thats rubbish! It’s make-believe unless you can actually present evidence for those other species. I could give a crap if an author *thinks* there are more species to be found – speculation is different than evidence.

“…diversity is a human artifact in the first place: whether we are counting “species-level” or “generic-level” diversity”

Which is why we shouldn’t be counting species-level or genus-level diversity. We should be looking at lineage diversity, which is not a human construct. Admittedly we will never know the *true* phylogeny, but thinking in terms of lineage diversity gets us the closest to reality, and is infinitely scalable.

“…the choice of whether or not two individuals belong to the same species, or to the same genus, is ultimately a matter of judgement and taste rather than of fact.”

Only if you disregard phylogeny. If you want you taxonomy to reflect phylogeny, you are at least constrained to make your taxa monophyletic. Of course there are still subjective decisions (for example – should the most basal member of the clade be put in a separate genus?), but that is exactly why we should get rid of Linnean ranks altogether, including genera and species.

“…apparently objective rules like “there is no reason to create a new genus if the existing genus is monophyletic” just make things worse.”

I did say it was my opinion, so don’t mischaracterize my position; I never claimed it was an objective rule. You also are making an unsupported statement – explain why it makes things worse. In my taxonomic framework, lineages and monophyly matter, not Linnean binomens, so I think this rule makes things better!

Yikes! A debate in the comment-thread. Well, I really don’t want to get sucked into this (although maybe I’ll take it on properly as an SV-POW! article some time), but to respond very quickly:

“Mike said: “…the former implies more diversity because it expresses the author’s feeling that there may well be a whole complex of new species out there.””

“Thats rubbish! It’s make-believe unless you can actually present evidence for those other species. I could give a crap if an author *thinks* there are more species to be found – speculation is different than evidence.”

Yes, speculation is different from evidence. That doesn’t make it worthless. I would take the speculation of someone who’s been working on sauropods for twenty years as informed speculation, and that is worthy of respect. Not slavish obedience, obviously, but simply ignoring it is dumb. The argument from authority is not conclusive, but it is an argument nonetheless.

In any case, remember that your original claim, which I was responding to, was “A new monotypic genus vs a new species in an existing genus express exactly the same amount of diversity”. No, they _express_ different opinions on diversity. That they do not prove those opinions true is a completely orthogonal matter.

““…diversity is a human artifact in the first place: whether we are counting “species-level” or “generic-level” diversity”

Which is why we shouldn’t be counting species-level or genus-level diversity. We should be looking at lineage diversity, which is not a human construct. Admittedly we will never know the *true* phylogeny, but thinking in terms of lineage diversity gets us the closest to reality, and is infinitely scalable.”

Tell me more of this lineage diversity — it’s new to me. I hope you don’t just mean counting the branch-points on the cladogram, since any fully resolved cladogram produced from an analysis of n OTUs will necessarily produce n-1 branch-points.

““…the choice of whether or not two individuals belong to the same species, or to the same genus, is ultimately a matter of judgement and taste rather than of fact.”

Only if you disregard phylogeny.”

No, always. Given a phylogeny in which OTUs A and B are recovered as sisters, it is always a matter of judgement whether we consider them members of the same species, species of the same genus or separate genera.

““…apparently objective rules like “there is no reason to create a new genus if the existing genus is monophyletic” just make things worse.”

I did say it was my opinion, so don’t mischaracterize my position; I never claimed it was an objective rule.”

Unless it is applied as an objective rule, what is the point of it? Surely the whole purpose (if any) of such a rule is to provide an objective means of deciding these matters?

“You also are making an unsupported statement – explain why it makes things worse.”

Consider the topology (Malawisaurus (Cedarosaurus (Brachiosaurus Giraffatitan))). Applying your “rule”, since Brachiosaurus is monophyletic there is no need to introduce Giraffatitan so we roll it into Brachiosaurus. But now Brachiosaurus is still monophyletic, so there is no reason its new sister taxon Cedarosaurus should not also be considered Brachiosaurus. But now there is no reason why its new sisten taxon Malawisaurus should not be rolled in. And so on, until all of Animalia, and then Plantae, becomes subsumed in Brachiosaurus. (Which come to think of it isn’t such a bad idea.)

Mike said: “Yikes! A debate in the comment-thread. Well, I really don’t want to get sucked into this (although maybe I’ll take it on properly as an SV-POW! article some time)”

I think a blog post which is a taxonomic philosophy debate would be a lot of fun! We’d need a referee, and focused point/counterpoint questions to respond to though.

“Yes, speculation is different from evidence. That doesn’t make it worthless.”

All true, but I don’t think taxonomy should be speculative. It should be based on hypotheses backed up with evidence.

“Tell me more of this lineage diversity — it’s new to me. I hope you don’t just mean counting the branch-points on the cladogram”

No, but it is counting the branches themselves on a time calibrated cladogram – for whatever time slice you are interested in. But you don’t need a cladogram – if you hypothesize anagenesis between two taxa (well, I would keep them as one taxon myself), then you would only count it as 1 unit of diversity because they constitute a single lineage. Taxic diversity would count them as 2 units, albeit in different time bins. And yes, I know, cladograms can’t deal with anagenesis effectively, so everyone has to be smart about how they use their data. But then again, I don’t think anagenesis can ever be confirmed in the fossil record, only disproved.

“Given a phylogeny in which OTUs A and B are recovered as sisters, it is always a matter of judgement whether we consider them members of the same species, species of the same genus or separate genera.”

Sure, what rank we give them is subjective, but we identify them as separate taxa nonetheless.

“But now there is no reason why its new sisten taxon Malawisaurus should not be rolled in. And so on, until all of Animalia, and then Plantae, becomes subsumed in Brachiosaurus.”

Of course this example is absurd – you’re setting up a straw man. With rankless taxonomy, you still name clades, so one can name the node of altithorax+brancai, all the brachiosaurids, etc. And if you insist on retaining the binomen, you just combine the terminal taxon name (“species”) with the closest named node. This has the added advantage that if you don’t know exactly where a taxon fits within the phylogeny, you don’t have to shoehorn it into a genus. For example, you could refer to “Brachiosauridae: brancai”, or “Somphospondyli: dixeyi”.

Sometimes don’t you just feel like naming your own genus? I do. Other people’s genera feel so constraining. I don’t know how the entomologists stand it, their species packed in cheek-by-jowl (or whatever it is bugs have) like Peeps in the wrapper.

Consider the topology (Malawisaurus (Cedarosaurus (Brachiosaurus Giraffatitan))). Applying your “rule”, since Brachiosaurus is monophyletic there is no need to introduce Giraffatitan so we roll it into Brachiosaurus. But now Brachiosaurus is still monophyletic, so there is no reason its new sister taxon Cedarosaurus should not also be considered Brachiosaurus. But now there is no reason why its new sisten taxon Malawisaurus should not be rolled in. And so on, until all of Animalia, and then Plantae, becomes subsumed in Brachiosaurus. (Which come to think of it isn’t such a bad idea.)

There might be a small problem of priority here …

But I don’t see how lineage diversity helps on the lower end. If two specimens indeed are conspecific in the sense of the BSC, they don’t represent different lineages, do they?

Thinking about sauropods and stegosaurs reminds me of a question I’ve wondered about for a long time:

In the Morrison formation, there were *several* sauropod species as well as a stegosaur or two, Camptosaurus, etc. How was there enough plant matter to support populations large enough to breed of all these big to superhuge herbivores?

Hey, I’m not a paleontologist, but rather a cello teacher, so my scientific credibility isn’t much, but a couple comments…

First off, I notice that many species of dinosaur, and other vertebrates, developed a longer neck as a balance for a particularly long or heavy tail. For an example, think of Mamenchisaurus. Who didn’t think that its ridiculously long neck made no sense, until its own “thagomizer” was discovered? I figure that, as a stegosaur, Miragaia didn’t need its long, heavily plated neck for feeding purposes, as much as to balance out the as-yet-unseen, but surely impressive tail armament. Does that make sense?

& William Miller, the herds of giant sauropods were probably migratory, likely across as much of the Earth’s landmass as they could get to. This would explain the Barosaurs and Brachiosaurs found in Africa, and the rapid spread of sauropod genuses across the continents in the Jurassic. And as for why they seem to be biggest in North America (along with everything else!), I suspect the warm, highly seasonal climate would favor larger species. Or, perhaps the Morrison Formation was used as a sort of elephant graveyard, with the older (and larger) sauropods congregating together to die, or mate, lay eggs, or some other function, which would make it more probable to find their remains piled up in giant bonebeds, rather than scattered across the globe. This is all speculation, but it makes more ecological sense.

Hi, John, thanks for joining us! I’m afraid this balance idea won’t fly, though: the Mamenchisaurus thagomizer is a pretty poor piece of kit, and would have had a negligible effect on the tail’s mass and moment — it would be a rounding error compared with that gigantic neck, all the more so given that the tail of Mamenchisaurus is proportionally very short. See the photo at https://svpow.wordpress.com/2008/01/17/how-big-was-hudiesaurus/

I’m afraid your thoughts on migration don’t fit the evidence, either: the faunal similarity between the Morrison Formation in the USA and the Tendaguru Formation in Tanzania have been overplayed in the past, but recent work on the sauropods, theropods and ornithopods of the Tendaguru shows that in fact there are NO genera represented in both formations — for example, the African “Barosaurus” material is now recognised as representing two endemic genera, Tornieria and Australodocus. By the Late Jurassic, North America and Africa were separated by the young Atlantic, so migration between them would not have been possible.

Sorry to be so negative, but it’s great that you’re thinking about this stuff. Welcome aboard!

If two specimens indeed are conspecific in the sense of the BSC, they don’t represent different lineages, do they?

Of course they can. Take the Galápagos ground finches: three lineages that can, and occasionally do, have fertile offspring with each other, but in dry years the hybrids are selected against, so the lineages stay distinct and are probably universally considered three species, even though under the BSC they are just one.

Also remember that interfertility is a plesiomorphy. It gets lost at different points in a tree, but can be retained by pretty disparate branches. Cases like that are known.

[…] to ornithischians. It was difficult to keep a tight lip back in February 2009 when the long-necked stegosaur Miragaia longicollum was published. Like the WWD diplodocoids, Miragaia was given a non-extended cervico-dorsal junction and extended […]

[…] to evolve shorter necks, I would think. And I see that your critter has a neck almost as short as Brachytrachelopan, and seems from the limb proportions to be a diplodocid. I’m curious, did you start with the […]

Mike Taylor said: “the faunal similarity between the Morrison Formation in the USA and the Tendaguru Formation in Tanzania have been overplayed in the past, but recent work on the sauropods, theropods and ornithopods of the Tendaguru shows that in fact there are NO genera represented in both formations”

Are Dryosaurus altus and Dysalotosaurus lettowvorbecki separate again? If so, since when?

Well, it’s not an open-and-shut case, but AFAIK the most recent published work on the African species is Witzmann et al. (2008), which used the name Dysalotosaurus throughout and briefly discussed the possibly synonymy as follows (p. 1150):

Pompeckj (1920) formally diagnosed the new genus and species Dysalotosauruslettowvorbecki based on the vast collection from bonebed dy. Galton (1977) transferred the East African species D. lettowvorbecki to the North American genus Dryosaurus from the contemporaneous Morrison Formation. Most recent workers accepted this synonymy (Norman, 2004, and references therein), but given that both the Dryosaurus and the Dysalotosaurus samples consist of juvenile individuals (Chinsamy, 1995; Padian et al., 2006) and that both taxa show more differences than listed by Galton (1981, 1983), the congeneric status of both species is far from being proven.

Even later post now, but I just went over some old stuff from Glut’s original encyclopedia and realized that the holotype of Dacentrurus (BMNH 46103) was listed as including “three cervical and 14 dorsal vertebrae…left humerus, radius, ulna, carpus, metacarpus, a phalanx, ilia and ischia, right pubis” among its contents.

Almost all the elements I just mentioned overlap with the holotype of Miragaia, despite numerous statements that there are no overlapping elements between the two holotypes. I do not mean to say that I know more than the authors on any of this, but I would like to know why there is such an apparent discrepancy. Are all those elements non-diagnostic?

I just read over my previous comment and realized that it sounded overly confrontational, a lot more hostile than I was hoping for it to sound. Sorry, everyone. I just wanted to know why Owen and Maidment seemed to have different ideas of what was in the Dacentrurus holotype and would like that clarified to ease confusion; I did not intend to accuse people of Bad Things.